The Problems Associated With Zebra Mussels
Accidentally introduced into the United States in the mid-1980's, zebra mussels (Dreissena polymorpha) have become a major threat to U.S. inland fresh water supplies. Zebra mussels have now spread throughout the Great Lakes region and as far south as Louisiana, and could cause more than $2 billion in economic damage by 1999.
Zebra mussels rapidly establish colonies on hard underwater surfaces, such as water intake pipes, boat hulls, and commercial fishing nets, forming layers up to eight inches thick (Wisconsin Sea Grant (WSG)). They can cluster together in colonies which can exceed a hundred thousand per square meter. A mature female can produce up to 40,000 eggs in one season (New York Sea Grant (NYSG)). Although a few species of fish and diving ducks eat the mussels, natural predators in North America are insufficient to control their growth.
Zebra mussels clog water intake pipes, diminishing the flow rate of water and adversely affecting industries, such as power plants, which rely on a continuous supply of intake water. Plants drawing water from infested areas have experienced 50% declines in intake efficiency (Nalepa and Schloesser, 1993). Recent studies also indicate that zebra mussels accelerate corrosion of iron and steel structures. The zebra mussel problem is particularly acute in the Great Lakes region, where about 655 billion gallons of water are withdrawn each day for use by more than 25 million people and hundreds of industries and power plants (NYSG).
Zebra mussels also significantly impact municipal drinking water supplies. In addition to clogging intake pipes, zebra mussels feed heavily on phytoplankton, dramatically increasing water clarity and promoting aggressive growth of aquatic weeds. This has led to taste and odor problems in drking water supplies. Drinking water supplies are further harmed from the putrefying flesh which occurs when zebra mussels die within intake pipes.
Zebra mussels also cause highly damaging ecological effects. Zebra mussel feeding has reduced some forms of phytoplankton, the base of the food chain, by as much as 80% (WSG). The reduction of plankton at the food chain base diminishes energy available for fish production, which is likely to reduce fish yields. The consumption of phytoplankton and related filtering of water by zebra mussels has increased water clarity in Lake Erie by up to 600% (WSG), altering the physical and chemical environment. As noted, one effect has been increased growth of aquatic weeds. Increased weed growth favors some fish species, such as sunfish, that prefer to spawn and hide in weed beds. However, wildlife biologists fear that increased water clarity will lead to the collapse of the $900 million walleye fishery, because the walleye favors turbid water (GLERL, 1994). In addition, zebra mussels have also nearly eliminated several native clam species from parts of the Great Lake region.
Zebra mussels also mobilize toxins from sediments into the food chain. Zebra mussels are capable of accumulating approximately 10 times more PCBs and other toxic contaminants than native mussels (WSG). They ingest these toxins in their fatty tissues when they eat algae to which toxins have been sorbed. When fish or birds eat these mussels, these toxic compounds then pass into the food chain in concentrated volumes.
In addition, zebra mussels help to promote growth of certain blue-green algae known as Microcystis, which are toxic to fish and cause gastrointestinal problems in humans. Zebra mussels tend to avoid Microcystis, while aggressively consuming less toxic species. This selective feeding has resulted in Microcystis blooms in Lake Erie and surrounding waters (NOAA, 1996).
Another related threat, the quagga mussel (Dreissena bugensis), has also recently been identified. Quagga mussels are similar to zebra mussels, but are found more than three times deeper in fresh water than zebra mussels. While zebra mussels are generally limited to shallow waters, quagga mussels pose a threat to boats and equipment located in much deeper waters.
A 1994 survey of electric generating power plants, municipal water systems and industrial water users in the Great Lakes region found that the average facility had expended over $430,000 on zebra mussel prevention and control as of 1994. Private utilities had spent an average of $869,000 per facility on control (Hushak, 1995). These amounts have undoubtedly increased substantially since 1994. The City of Baltimore has targeted $4.6 million to install zebra mussel control systems to protect its drinking water supplies, even though zebra mussels have not yet invaded Maryland (Wilkinson, 1997).
The most common treatment for zebra mussels is chlorination. Chlorine is effective against zebra mussels at low doses, and is typically applied continuously at concentrations varying between 0.5 mg/l to 2-3 mg/l (depending on time of season and whether chlorine is being applied to prevent or treat mussel infestation). However, chlorine can produce a wide variety of carcinogenic and other toxic by-products. Chlorine typically cannot be applied directly in fresh water lakes due to environmental concerns. Because of environmental problems, the Ohio Sea Grant (OSG) has stated that it no longer recommends chlorine disinfection to treat zebra mussels (OSG, 1994).
The Problems Associated With Ballast Water
An estimated 20 billion gallons of ballast water enters U.S. ports annually (Wright et al., 1998). Many major aquatic nuisances arrived in the U.S. in ballast water, including zebra mussels and other species. These non-indigenous species sometimes cause severe economic losses, damage or destroy ecosystems, and cause a loss of biodiversity. Some organisms are capable of causing death or illnesses.
The damages caused by the zebra mussels are documented above. Other exotic species have also caused serious economic and ecological damage. For instance, the shore crab, Hemigrapsus sanguineas, was first discovered on the U.S. Atlantic Coast in 1988 in New Jersey. It has now spread from Massachusetts to North Carolina. The exotic marine/estuarine brown mussel has displaced native mollusks and threatens mangrove communities in the gulf of Mexico. (GLPANS, 1996).
The seaweed species Sargassum muticum was introduced into the U.S. West Coast in the 1940's and is now found from British Columbia to California. Its efficient dispersal methods and fast growth allow it to compete effectively with native species for space and light. It supports a very different group of organisms than native seaweed species, thus transforming entire ecosystems (Giver, 1999).
Hydrilla, an introduced plant species, has become a major pest. In Florida, the area infested by hydrilla doubled between 1994 and 1996, to 100,000 acres of inland water. The state spends $14 million annually to control this plant, which affects several other parts of the U.S. as well (GLPANS, 1996).
Several introduced species pose health risks. The Chinese mitten crab, introduced into San Francisco Bay, is host to the oriental lung fluke, a parasite affecting humans and other mammals (Id.). A South American human cholera strain was found in ballast tanks in the port of Mobile, Ala. in 1991 (Id.).
Approximately 230 exotic species are now established in the San Francisco Estuary, with approximately four new species becoming established every year (Cohen, 1999). A recent study indicated that more than 90 percent of 70 vessels surveyed in the Chesapeake Bay carried live organisms in their ballast, including barnacles, clams, mussels, microscopic plants and animals, and fish (GLPANS, 1996).
Present controls for ballast water consist largely of exchanging fresh water for salt water at sea, in order to ninimize survival of salt-intolerant species. However, this exchange is frequently incompletely performed and may be waived entirely in unfavorable conditions.
International agreements governing regulation of ballast water are presently being drafted. Juglone is an ideal candidate for treatment of ballast water, because it is inexpensive, effective against a broad range of potential aquatic nuisance species and other pest species at extremely low concentrations, and can be inexpensively treated to be rendered non-toxic prior to being discharged. (Furthermore, ocean waters are alkaline, with a normal pH of about 8.2-8.3. Since juglone rapidly biodegrades in alkaline conditions, it would quickly break down even if it did enter ocean waters).
General Background
Clark et al. (1990) reported that the black walnut was historically used in the southern U.S. to treat ringworm. They found that juglone demonstrated significant antifungal activity against certain types of fungi. Auyong et al. (1963) reported that juglone had a depressant effect on goldfish and several species of rodents.
In 1971, the U.S. Interior Department received a patent entitled "Method of Fish Culture" based on the use ofjuglone. They found that juglone killed a wide variety of undesired fish species at very low concentrations and then rapidly biodegraded so that it could be used to restock a pond with new fish species shortly thereafter (DOI, 1971, U.S. Pat. No. 3,602,194).
The U.S. Department of the Interior reported in 1970 that juglone demonstrated LD.sub.50 results ranging from 0.027 to 0.088 ppm (parts per million) against nine fish species from seven genera. The government researchers also found that there was a very narrow range between the juglone levels which permitted complete survival and those that resulted in complete mortality (DOI, 1971).
They found that juglone biodegraded so rapidly that the pond could be safely restocked with new fish in between 10 and 60 days. They recommended a treatment level of 100 to 300 ppb ofjuglone for maximum effectiveness in clearing fish ponds. (By way of comparison, chlorine is presently used in continuous treatment of zebra mussels at levels varying between 0.5 mg/l to 2-3 mg/l.) The government researchers also found that juglone demonstrated consistent toxicity across a range of temperatures (DOI, 1971).
Another patent has been issued based on the use of a derivative from black walnut husks as a means of harvesting earthworms. The black walnut derivative is mixed with water and then applied to the soil. Application of this mixture causes earthworms to come out of the soil quickly, where they can be gathered as fishing bait. (U.S. Pat. No. 4,178,711 (1979)).
Juglone has not been used heretofore to treat infestations of zebra mussels, quagga mussels or any other aquatic pest. The standard current treatment for zebra mussels is chlorination. Chlorine is commonly applied as follows:
(a) low-level continuous chlorination at 0.5 mg/l
(b) 2-3 mg/l applied continuously, 3-4 weeks following zebra mussel settlement
(c) 2-3 mg/l intermittently, 2-8 hours/day during the zebra mussel reproductive season (May-Nov)
Regulatory discharge limits may affect the above amounts, e.g., maximum daily discharge of 2 hours, with concentration limitations of 0.2 mg/l for 30 days average or 0.5 mg/l on a daily basis unless dechlorination is used. Regulatory limits vary from state to state.
As noted earlier, chlorine can produce a wide variety of carcinogenic and other toxic by-products. We conducted a toxicology assay for chlorine on fathead minnows and obtained an LD.sub.50 of 0.12 ppm. Not surprisingly, this result is indicative of strong toxicity.
Several alternatives to chlorine have been suggested. Fisher et al (1991) tested five potassium salts, KI, K.sub.2 SO.sub.4, K4P2O7, KCl, and KH2PO4, and found that they had LD.sub.50 's against zebra mussels ranging from a low of 92 mg/l to a high of 226 mg/l. Claudi and Macke (1994) reported that ozone caused approximately 50% mortality in adult zebra mussels exposed to residual ozone at levels of 2 mg/l for four days, and 100% mortality after seven days.
Some proprietary chemicals for the treatment of zebra mussels are already available. Clam-Trol, produced by Betz Chemicals, and H130 produced by Calgon Corp., are registered for use in the United States. However, both compounds are acutely toxic to fish and other aquatic organisms and are believed to be quite persistent in the environment (Claudi and Macke, 1994). Furthermore, Fisher et al (1994) found that while they were effective against larvae at less than 0.2 ug/ml, these compounds had 24 hour LD.sub.50 s against 5-8 mm adults in excess of 10 mg/l. Two chlorine compounds, Bayer 73 (Bayluscicide) and Sal I, reportedly had 24 hour LD.sub.50 's against adults of less than 60 ug/l. Both compounds are highly toxic, however. Rotenone gave an LD.sub.50 of 0.2 ppm against early stage zebra mussel larvae, and surprisingly was more effective against adults (Fisher et al, 1994). One other natural product, endod, derived from the soapberry plant, has also been promoted as a treatment for zebra mussels. However, endod requires at least 5-20 ppm to kill zebra mussels (University of Toledo). Some non-chemical treatments are also reportedly being developed, such as UV treatment. These treatments are reportedly still prohibitively expensive. To date, none of these existing alternative treatments seems likely to replace chlorination as the standard treatment for zebra mussels.
In addition, juglone has not been used before now to clean ballast water of aquatic pests or other non-desirable non-indigenous aquatic species. As noted above, open-sea ballast water exchange is the primary means of ballast water treatment at present. This method is insufficient in preventing infestations. Gluteraldehyde has been proposed as an alternative treatment. However, gluteraldehyde requires ship-board pumping and metering equipment. It requires concentrations of 5-150 ppm to be effective and is not feasible in many situations. Other contemplated solutions, such as filters or UV radiation are complex to use, more expensive, have also proven only partially effective, and have little effect in certain areas such as the sludge layer underlying the ballast.